Seon Joon Kim

Highly Enhanced Gas Adsorption Properties in Vertically Aligned MoS2 Layers

In this work, we demonstrate that gas adsorption is significantly higher in edge sites of vertically aligned MoS2 compared to that of the conventional basal plane exposed MoS2 films. To compare the effect of the alignment of MoS2 on the gas adsorption properties, we synthesized three distinct MoS2 films with different alignment directions ((1) horizontally aligned MoS2 (basal plane exposed), (2) mixture of horizontally aligned MoS2 and vertically aligned layers (basal and edge exposed), and (3) vertically aligned MoS2 (edge exposed)) by using rapid sulfurization method of CVD process. Vertically aligned MoS2 film shows about 5-fold enhanced sensitivity to NO2 gas molecules compared to horizontally aligned MoS2 film. Vertically aligned MoS2 has superior resistance variation compared to horizontally aligned MoS2 even with same surface area exposed to identical concentration of gas molecules. We found that electrical response to target gas molecules correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. Density functional theory (DFT) calculations corroborate the experimental results as stronger NO2 binding energies are computed for multiple configurations near the edge sites of MoS2, which verifies that electrical response to target gas molecules (NO2) correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. We believe that this observation extends to other 2D TMD materials as well as MoS2 and can be applied to significantly enhance the gas sensor performance in these materials.

Role of 1D metallic nanowires in polydomain graphene for highly transparent conducting films.

Upon the addition of silver nanowires (AgNWs), the electrical conductance of graphene film is improved. According to the films optical birefringence, as shown by studies using liquid crystals (LCs), the improvements do not result from the chemical doping properties of the AgNWs; instead, they arise because the AgNWs facilitate connections among the domains in the graphene film. This is further supported by the films Dirac point voltage, Raman spectra, and electrical resistance.

Complete magnesiothermic reduction reaction of vertically aligned mesoporous silica channels to form pure silicon nanoparticles

Owing to its simplicity and low temperature conditions, magnesiothermic reduction of silica is one of the most powerful methods for producing silicon nanostructures. However, incomplete reduction takes place in this process leaving unconverted silica under the silicon layer. This phenomenon limits the use of this method for the rational design of silicon structures. In this effort, a technique that enables complete magnesiothermic reduction of silica to form silicon has been developed. The procedure involves magnesium promoted reduction of vertically oriented mesoporous silica channels on reduced graphene oxides (rGO) sheets. The mesopores play a significant role in effectively enabling magnesium gas to interact with silica through a large number of reaction sites. Utilizing this approach, highly uniform, ca. 10 nm sized silicon nanoparticles are generated without contamination by unreacted silica. The new method for complete magnesiothermic reduction of mesoporous silica approach provides a foundation for the rational design of silicon structures.

The effects of the crystalline orientation of Cu domains on the formation of nanoripple arrays in CVD-grown graphene on Cu

Defect structures such as boundaries, ripples and wrinkles in graphene have been considered as main causes reducing the electrical properties of graphene. Among them, the formation of a periodic nanoripple array and surface roughening intrinsically occurs as graphene grows on the surface of a metal catalyst during chemical vapor deposition, which results in anisotropic charge transport and limits the possible sheet resistance. In this study, we observed that among the various growth factors, the crystalline orientation of Cu domains can play an important role in the occurrence of periodic surface roughening. With the exception of Cu (111) domain, the surfaces of Cu domains are considerably rippled to a particular direction with abundant terrace structure and step edges. Such ripples occur to relax the strain from a large lattice mismatch between graphene and Cu lattice at a high temperature during the CVD process, which remain as rippled regions of graphene after wet transfer. However, a relatively flat surface is observed in the graphene transferred from hexagonal Cu (111) domain. Additional conductivity mapping also reveals that graphene from Cu (111) domain shows highly homogeneous current distribution. On the other hand, degraded conductivity on rippled regions introducing anisotropic transport of current is observed in the graphene from Cu domains except Cu (111) domain. We believe that current observation can contribute to the preparation of graphene with flat structure simply by controlling the crystalline orientation of Cu.

Direct observation of molybdenum disulfide, MoS2, domains by using a liquid crystalline texture method.

Because the properties of molybdenum disulfide (MoS2) are strongly influenced by the sizes and boundaries of its domains, the direct visualization of large-area MoS2 domains is one of the most important challenges in MoS2 research. In the current study, we developed a simple and rapid method to observe and determine the boundaries of MoS2 domains. The technique, which depends on observations of nematic liquid crystal textures on the MoS2 surface, does not damage the sample and is not limited by domain size. Thus, this approach should significantly aid not only efforts aimed at gaining an understanding of the relationships between grain boundaries and properties of MoS2 but also those focusing on how domain sizes are controlled during large-area synthesis.

Combining the silver nanowire bridging effect with chemical doping for highly improved conductivity of CVD-grown graphene films

Here, the conductivity of graphene films was significantly enhanced, with only a negligible loss in optical transmittance, by controlling the intrinsic electrical properties and domains through a combination of chemical doping and the deposition of a few one-dimensional (1D) silver nanowires (AgNWs). Ultraviolet photoelectron spectrophotometry (UPS) results and Raman spectra showed that the transparent conducting performance of the CVD-grown graphene film can be attributed to the chemical doping effect and the electric pathways provided by the AgNW bridging between graphene domains. Importantly, it was found that the transparent conducting performance of the AgNW modified CVD graphene films was significantly influenced by the types and sequence of chemical doping. Various combinations were investigated, including: (i) Au pre-treatment and then AgNW deposition, (ii) AgNW deposition and then Au post-treatment, (iii) acid (HNO3) pre-treatment and then AgNW deposition, and (iv) AgNW deposition and then acid (HNO3) post-treatment. Electrical conductivity and transmittance results showed that the combination of Au pre-treatment and subsequent AgNW deposition onto the graphene films is the most effective way to enhance the conductivity of graphene films for a variety of optoelectronic device applications, demonstrating a large reduction in the sheet resistance of the graphene film (ΔRs ≈ 80%) without significant loss of transmittance. This is due to independent evolution of the chemical doping effect of Au and the bridging effect of the AgNWs on the poly-domain graphene. For deposition of AgNWs and subsequent Au post-doping, on the other hand, transmittance was reduced by more than 5% after Au post-treatment of the graphene films on which AgNWs had already been deposited.

Ultraclean transfer of CVD-grown graphene and its application to flexible organic photovoltaic cells

We demonstrate a PMMA reverse transfer method, where a PMMA/graphene bilayer was reversely transferred onto target substrates, to better control both contamination and crack formation relative to conventional approaches. Based on this novel transfer process, the graphene sheet resistance was greatly reduced by about 50% at the same transmittance, exhibiting ∼200% higher efficiency when applied in a solar cell as the anode.

Metallic Ti3C2Tx MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio

Achieving high sensitivity in solid-state gas sensors can allow the precise detection of chemical agents. In particular, detection of volatile organic compounds (VOCs) at the parts per billion (ppb) level is critical for the early diagnosis of diseases. To obtain high sensitivity, two requirements need to be simultaneously satisfied: (i) low electrical noise and (ii) strong signal, which existing sensor materials cannot meet. Here, we demonstrate that 2D metal carbide MXenes, which possess high metallic conductivity for low noise and a fully functionalized surface for a strong signal, greatly outperform the sensitivity of conventional semiconductor channel materials. Ti3C2Tx MXene gas sensors exhibited a very low limit of detection of 50-100 ppb for VOC gases at room temperature. Also, the extremely low noise led to a signal-to-noise ratio 2 orders of magnitude higher than that of other 2D materials, surpassing the best sensors known. Our results provide insight in utilizing highly functionalized metallic sensing channels for developing highly sensitive sensors.

Large-Area Buckled MoS2 Films on the Graphene Substrate.

In this study, a novel buckled structure of edge-oriented MoS2 films is fabricated for the first time by employing monolayer graphene as the substrate for MoS2 film growth. Compared to typical buckling methods, our technique has several advantages: (1) external forces such as heat and mechanical strain are not applied; (2) uniform and controllable buckling over a large area is possible; and (3) films are able to be transferred to a desired substrate. Dual MoS2 orientation was observed in the buckled film where horizontally aligned MoS2 layers of 7 nm thickness were present near the bottom graphene surface and vertically aligned layers dominated the film toward the outer surface, in which the alignment structure was uniform across the entire film. The catalytic ability of the buckled MoS2 films, measured by performing water-splitting tests in acidic environments, shows a reduced onset potential of -0.2 V versus reversible hydrogen electrode (RHE) compared to -0.32 V versus RHE for pristine MoS2, indicating that the rough surface provided a higher catalytic activity. Our work presents a new method to generate a buckled MoS2 structure, which may be extended to the formation of buckled structures in various 2D materials for future applications.

Key growth parameters affecting the domain structure of chemical vapor deposition (CVD)-grown graphene on nickel

The domain structures of multilayer graphene grown on Ni using a chemical vapor deposition (CVD) process were characterized using an optical birefringence visualization method. Unlike graphene grown on Cu, the Ni surface topography, rather than the Ni crystal orientation, significantly influenced the graphene domain size and structure. Smoother surfaces yielded larger domains. The graphene domains in the inner layers closer to the substrate in multilayer graphene were smaller than the outer layers on the top surface.